Article
pubs.acs.org/est
Measured Wavelength-Dependent Absorption Enhancement of
Internally Mixed Black Carbon with Absorbing and Nonabsorbing
Materials
Rian You,†,‡ James G. Radney,† Michael R. Zachariah,†,‡ and Christopher D. Zangmeister*,†
†
Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
‡
S Supporting Information
*
ABSTRACT: Optical absorption spectra of laboratory generated aerosols consisting
of black carbon (BC) internally mixed with nonabsorbing materials (ammonium
sulfate, AS, and sodium chloride, NaCl) and BC with a weakly absorbing brown
carbon surrogate derived from humic acid (HA) were measured across the visible to
near-IR (550 to 840 nm). Spectra were measured in situ using a photoacoustic
spectrometer and step-scanning a supercontinuum laser source with a tunable
wavelength and bandwidth filter. BC had a mass-specific absorption cross section
(MAC) of 7.89 ± 0.25 m2 g−1 at λ = 550 nm and an absorption Ångström exponent
(AAE) of 1.03 ± 0.09 (2σ). For internally mixed BC, the ratio of BC mass to the total
mass of the mixture was chosen as 0.13 to mimic particles observed in the terrestrial
atmosphere. The manner in which BC mixed with each material was determined from
transmission electron microscopy (TEM). AS/BC and HA/BC particles were fully
internally mixed, and the BC was both internally and externally mixed for NaCl/BC
particles. The AS/BC, NaCl/BC, and HA/BC particles had AAEs of 1.43 ± 0.05, 1.34 ± 0.06, and 1.91 ± 0.05, respectively. The
observed absorption enhancement of mixed BC relative to the pure BC was wavelength dependent for AS/BC and decreased
from 1.5 at λ = 550 nm with increasing wavelength while the NaCl/BC enhancement was essentially wavelength independent.
For HA/BC, the enhancement ranged from 2 to 3 and was strongly wavelength dependent. Removal of the HA absorption
contribution to enhancement revealed that the enhancement was ≈1.5 and independent of wavelength.
■
INTRODUCTION
EAbs, λ =
Black carbon (BC) is produced from the incomplete
combustion of biomass and fossil fuels, and consists of layered
graphitic carbon monomers 20−40 nm in diameter1 that are
aggregated into more complex structures that strongly absorb
light across the visible spectrum.2 It has been estimated that BC
is the second largest contributor to global warming after CO2.3
BC aggregates are often internally and/or externally mixed with
other species (sulfate, nitrates, carbonaceous organics, etc.).4−12
The resulting particle morphology, referred to here as the
mixing state, can have BC surface bound, or partially or fully
embedded (engulfed) with BC either centrally (concentric) or
off-centrally (eccentric) located within the particle.8,13
Changes to the per particle absorption strength (i.e.,
absorption cross section) and spectral shape when other
atmospheric components coat, embed in or mix with BC have
drawn significant attention14−17 as absorption is typically
enhanced beyond the bare particle case. Consistent with
previous investigations,17,20,22 we define the wavelength
dependent absorption enhancement factor (EAbs,λ) as the ratio
of the absorption cross section (CAbs) of the embedded particle
(CAbs,embedBC,λ) to that of the bare BC particle (CAbs,BC,λ)
This article not subject to U.S. Copyright.
Published 2016 by the American Chemical
Society
CAbs,embedBC, λ
CAbs,BC, λ
(1)
where CAbs is defined as
CAbs =
αAbs
N
(2)
and N is the number concentration of particles. The quantity
αAbs represents the absorption coefficient and is the fractional
loss in light intensity due to absorption per unit propagation
distance.
Considering the size of particles and the thickness of
embedding material relevant in the atmosphere (i.e., comparable to or smaller than the wavelength of light), absorption
enhancement arises from the gradual stepwise refractive index
(RI) change between the medium (air), embedding material
and the absorbing material (BC in most cases); i.e. the
embedding material facilitates improved refractive index
matching between the medium and absorbing material thereby
Received:
Revised:
Accepted:
Published:
7982
March 24, 2016
June 16, 2016
June 30, 2016
June 30, 2016
DOI: 10.1021/acs.est.6b01473
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Environmental Science & Technology
Article
values ranging from 1.5 to ≈7.0.24,31,38−45 Additional
experimental studies are required to fully understand the
impact of BC embedded within BrC.
Direct in situ absorption measurements can be made using a
photoacoustic spectrometer. Ideally, absorption measurements
at many points spanning the visible portion of the spectral
window would be collected to elucidate the wavelength
dependence of embedded BC. In practice, four or fewer
wavelengths are typically used as a sufficiently intense source
(e.g., laser) is necessary to generate a measurable acoustic
signal; further, the use of multiple sources may require the use
of multiple acoustic resonators and data acquisition systems. It
is possible to equip a photoacoustic spectrometer with a broadband source such as an optical parametrical oscillator,46 a Hg
arc lamp,47 or supercontinuum laser48,49 to acquire stepscanned absorption spectra. In the present study seven points
were used to comprise a spectrum, although only two
wavelengths are required to fit eq 3, where it will reduce to
the explicit form used extensively in prior studies.50−53
The discrepancy between ambient field-based measurements
and computational-based predictions of absorption for spherical
systems of BC in different mixing states requires measurements
of well-characterized laboratory-based aerosol. This investigation describes absorption measurements of BC mixed with
nonabsorbing materials (ammonium sulfate and sodium
chloride) and a weakly absorbing BrC surrogate, humic acid
(HA); all of which are atmospherically relevant species. The
mass mixing ratio (χBC) defined as the ratio of BC mass to the
mass of the total mixture, was chosen as 0.13, similar to
particles collected in the terrestrial atmosphere.13 The
absorption spectra were measured using a photoacoustic
spectrometer coupled to a supercontinuum light source with
a tunable wavelength and bandwidth filter. The measured data
as a function of embedding material and mixing state were
compared to EAbs calculated using the T-matrix method.
behaving similar to an antireflective coating.18 The observed
enhancement is strongly dependent upon the size and
absorption strength of the absorbing material, embedding
thickness, and embedding material. Previous investigations have
referred to this refractive index matching as lensing whereby the
embedding material focuses light toward the absorbing
core.17,19,20 We note that while lensing may be an appropriate
descriptor in the geometric optics regime (particles much larger
than the wavelength of light) it is not appropriate to use at
these sizes.
For an absorbing core comprised of 10 nm aggregates
embedded in a 1 μm nonabsorbing shell, the EAbs,λ can be
greater than 10. However, for atmospherically relevant particles,
the EAbs,λ is more modest and ranges between 1.5 and 1.9 for
particles with shell diameters that are less than 1.6 times larger
than the core diameter.14 Ambient measurements show that the
EAbs,λ of BC can be up to 2.4.11,21−26 The discrepancy between
field measurements and theoretical calculations of internally
mixed, embedded particles can be computationally reduced
with the consideration of an eccentrically located single BC
core in an internally mixed embedded particle or by the
application of discrete dipole approximation method (DDA) or
multiple sphere T-matrix method (MSTM) to account for the
complex aggregate morphology and the interaction between
spherules.27−29 Moreover, it has been shown computationally
that the mixing state of BC has a great impact on the absorption
enhancement factor.30 In contrast to the ambient field-based
observations, laboratory measurements tend to coincide more
closely to E Abs,λ predicted by the concentric sphere
model.2,31−33 Previous laboratory studies have concentrated
on BC embedded in materials generated from the ozonolysis
products from α-pinene,34 glycerol and oleic acid,32 and dioctyl
sebacate,21 which can produce a BC core embedded in an
organic shell (core−shell model). These investigations
observed that the core−shell model can be adequately captured
from current theory, but are limited in relevance to particles
observed in many field studies.35 Further, in ambient studies
the embedding material must be physically removed to measure
the absorption by the BC core. Removal of the embedding
material is typically done by volatilization, although the efficacy
of removal remains questionable.25,36 To our knowledge,
however, no laboratory study has been reported on the
absorption enhancement of heterogeneous BC particles with
different mixing states in a controlled manner.
In addition to EAbs,λ, spectral shape can be affected by particle
mixing state. The absorption spectral dependence is commonly
described by the absorption Ångström exponent (AAE)
⎛ λ ⎞−AAE
CAbs, λ = CAbs,0⎜ ⎟
⎝ λ0 ⎠
■
MATERIALS AND METHODS
A schematic of the experimental setup used is shown in Figure
1. Aerosols were generated and conditioned before being
Figure 1. Experimental schematic used for the characterization of
aerosol size, mass, and optical absorption properties. Abbreviations:
differential mobility analyzer (DMA), aerosol particle mass analyzer
(APM), photoacoustic spectrometer (PA), and condensation particle
counter (CPC).
(3)
where CAbs,λ is the absorption cross section at wavelength λ.
The terms CAbs,0 and λ0 have been included as the values for a
reference wavelength; presently, λ0 = 550 nm for comparison to
prior work. The AAE for BC is typically quoted as being near
unity and assumes a wavelength independent refractive index
for particles <40 nm in diameter.37 Recent modeling has shown
that BC cores embedded within a nonabsorbing matrix (50 nm
embedding in a 300 nm BC core) can increase the AAE from
an initial value of 1.3 up to 1.6.17 BC can also be mixed or
embedded with weakly absorbing carbonaceous materials
derived from combustion of biomass and biofuels, termed
brown carbon (BrC). BrC is typically quoted as having AAE
passed to a differential mobility analyzer (DMA) and an aerosol
particle mass analyzer (APM) to select the mobility diameter
(Dp) and mass (mp), respectively. Optical absorption was
measured by a photoacoustic spectrometer (PA) that utilizes a
supercontinuum laser as its light source. A condensation
particle counter (CPC) was situated downstream to measure
aerosol number density. Using both mobility and mass
selection facilitates isolation and selection of particles bearing
a +1 charge. Care was used to ensure that only +1 particles
were selected by measuring particle extinction as a function of
mp as previously described in Radney and Zangmeister
(2016).54
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Aerosol Generation and Morphological Characterization. BC was generated from Cab-O-Jet 200 (Cabot Corp.,
20.03 wt % solids), a material generated from the combustion
of organic fuel stock.55 This material was chosen for its water
solubility and its spectroscopic and morphological similarities
to aged BC (see discussions in the Results section). Cab-O-Jet
in solution consists of dispersed monomers ≈30 nm in
diameter as evidenced by transmission electron microscopy
(TEM) images (see Supporting Information Figure S1).
Atomization of a Cab-O-Jet solution produces water droplets
containing multiple monomers. Upon drying, the monomers
combine and collapse into a structure that appears similar to
collapsed BC observed in the terrestrial atmosphere.56,57
A BrC surrogate (AAE > 4) consisting of humic acids (HA)
was prepared from Ful-Humix (Faust BioAg Inc., 50 wt %
humic acids) by dissolving 500 mg in 15 mL of DI H2O. The
solution was centrifuged for 30 min, and the supernatant was
decanted and collected and centrifuged for an additional 30
min. The supernatant was again collected, combined, and
subsequently filtered to remove any residual solids. The filtered
solution was then air-dried. The density of the HA was
measured in bulk (mass per unit volume) and calculated as the
average effective density from the mass-mobility scaling
relationship (see discussion in the Results section and the
Supporting Information). From the residual dried solid
material, a stock solution of 5 mg mL−1 was prepared for
absorption measurements. The nonabsorbing materials,
ammonium sulfate (AS, Sigma-Aldrich) and sodium chloride
(NaCl, Sigma-Aldrich) were used as-received and stock
solutions of 5 mg mL−1 were prepared.
To mimic BC containing aerosol measured in Mexico City
aerosol plumes,13 internally mixed particles were produced by
coatomization of a single aqueous solution containing both BC
and either AS, NaCl, or HA at a 0.13 BC mass fraction (χBC) in
a liquid jet cross-flow atomizer (TSI 3076, 30 psig); BC volume
fractions were 0.12, 0.15, and 0.11 for the AS/BC, NaCl/BC,
and HA/BC particles, respectively. At this pressure, ≈2.2 L
min−1 of flow is generated by the atomizer of which ≈0.5 L
min−1 was sampled and the remainder was exhausted into a
laboratory exhaust. Both Cab-O-Jet (BC) and HA required
extensive drying due to their hygroscopic nature and extra effort
was made to remove particle-bound water in order to avoid
additional absorption enhancement48 or potential photoacoustic response dampening58−61 from water adsorption.
After atomization, aerosols were passed through two SiO2
desiccation dryers (TSI 3062) and a Nafion drying tube
(PermaPure MD-700-48F-3) with the counter flow relative
humidity (RH) held at <5%. After drying, particles were size
selected by a DMA (TSI 3082) using 5 L min−1 sheath flow
and 0.5 L min−1 aerosol flow. The particles were then massselected using an APM as described previously.48
For TEM measurements particles were collected using an
electrostatic aerosol precipitator (TSI 3089) on TEM grids
(200-mesh copper grids embedded with lacey carbon film) at
−9.3 kV collection voltage. The morphology and mixing states
of the BC particles were imaged using a JEOL 2100 TEM at an
accelerating voltage of 200 kV.
Optical Measurements. Aerosol optical absorption spectra
were measured using a PA spectrometer as described in Radney
and Zangmeister (2015),48 which can operate across a range
spanning from visible to near IR (λ = 550 to 840 nm) using a
supercontinuum laser (NKT Photonics SuperK Extreme EXR15) and tunable wavelength and bandpass filter (NKT
Photonics SuperK Varia). The particles were illuminated by
an intensity modulated laser (via a mechanical chopper) at the
resonant frequency of the acoustic cavity (≈1640 Hz at 296 K
in ambient air).62 Absorbed optical energy is thermally reemitted generating a standing pressure wave (i.e., sound wave)
at the modulation frequency that was measured by a calibrated
microphone located at the resonator antinode, as described
previously.48 A lock-in amplifier (Stanford Research Systems,
SR830) was employed for phase-sensitive detection allowing
for the absorption coefficients to be calculated from
αAbs =
(x − x0)2 + (y − y0 )2
CcβmWpp
(4)
where x, y, Cc, βm, and Wpp are the in-phase and quadrature
signals measured by the lock-in amplifier, the acoustic cell
constant, the microphone responsivity, and the peak-to-peak
laser power, respectively. For the current system Ccβm is 0.187
V m W−1. The terms x0 and y0 have been included to account
for the lock-in amplifier’s small, but nonzero background signal
is measured when the laser is off.
Modeling BC Optical Properties. The optical properties
of bare BC and BC mixed with other components were
modeled using the multiple-sphere superposition T-matrix
method.63 BC aggregates were constructed using a diffusionlimited aggregation algorithm64 as described in the Supporting
Information. Calculated absorption of BC mixed with other
species (i.e., AS, NaCl, and BrC) assumed BC possessed a
similar morphology to bare BC, except that the number of
monomers was scaled relative to the mass ratio (0.13 mass
ratio). Two cases were investigated, fully and partially
embedded where either (1) a single host sphere was used
with the diameter identical to the Dp selected by the DMA, with
the BC aggregate’s center of mass overlapping that of the host
sphere, or (2) half of the monomers in the BC aggregate were
moved radially to the outside of the sphere and remained
attached, respectively.
■
RESULTS
Characterization of Bare BC. Bare BC aerosol had a
compact, spherical morphology comprised of 30 nm monomers. TEM images revealed monomers with discontinuous
onion-like fringes, consistent with flame generated soot and
indistinguishable from TEM images of aged soot in the
atmosphere;1 see Figure 2a. The measured mass-mobility
scaling exponent Dfm was 2.83 ± 0.01 consistent with the nearly
spherical shape observed in TEM images. The effective density
ρeff defined as
ρeff =
6mp
πDp3
(5)
was measured as 0.78 ± 0.05 g cm−3 over a Dp range between
100 and 300 nm. Figure 2b shows the measured CAbs at λ =
600, 700, and 800 nm of bare BC as a function of particle mass
spanning from 0.47 to 10.48 fg; Dp = 100−300 nm in 50 nm
increments. The linearity at each wavelength indicates that the
CAbs scales with particle mass (i.e., Rayleigh regime) across the
spectral region used in this investigation up to the pure 10.48 fg
BC particle. For comparison, the average internally mixed AS/
BC and NaCl/BC particles (see below) contained ≈1.4 and 1.7
fg of BC, respectively. Due to the wide mass range over which
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g−1 and the AAE is 1.03 ± 0.09 over the measured wavelength
range, similar to the average MAC and AAE for BC reported in
prior studies (all reported uncertainties are 2σ).19,66
The RI at 660 nm of bare BC was calculated as (1.77 ± 0.02)
+ (0.80 ± 0.02)i; see discussion in the Supporting Information.
A broad range of RIs for atmospheric BC are documented in
the literature.67−69 The RI of the bare BC measured in this
study is at the upper end of the previously published RI values
for atmospheric BC, but similar to other investigations for
particles with compact morphology.19,68 To validate the BC
parameters used in subsequent calculations, the absorption
spectrum of bare BC was calculated using the T-matrix
method29 using a refractive index of m = 1.77 + 0.80i and an
aggregate structure calculated using DLCA with k0 = 1.2, Df =
2.83, Nm = 218 and using the calculated BC RI (see the
Supporting Information). The BC RI was assumed to be
wavelength independent as discussed in the work of Bond and
Bergstrom.19 Using these assumptions, the T-matrix is able to
adequately capture the measured CAbs and AAE; see the solid
red line in Figure 2c.
Characterization of Embedded BC Mixing State. The
absorption spectrum of BC embedded in three atmospherically
relevant materials was measured for two nonabsorbing
materials (AS and NaCl) and a surrogate for brown carbon,
HA (i.e., Ångström exponent ≥ 4).17 BC particles can be
embedded with AS and/or other aerosol at a BC mass fraction
(χBC) ≈ 0.1.13 Particle mixing state and optical properties are
influenced by the interaction between BC and the embedding
material,13 drying rate,22 and history.70
In the present investigation, TEM images show structural
differences for AS/BC, NaCl/BC, and HA/BC, see Figure 3,
for 250 nm mobility diameter particles. AS/BC and HA/BC
formed spherical particles with BC aggregates fully embedded
within an AS or HA shell (Figure 3a and c, respectively). NaCl/
BC particles have a cubic NaCl shell with BC aggregates
protruding from the particle interior to its periphery (Figure
3b). Further evidence of the particle mixing state was
Figure 2. (a) TEM image of bare 250 nm BC particles used in study.
(b) BC absorption cross section (CAbs) as a function of particle mass at
λ = 600, 700, and 800 nm. (c) Absorption spectrum (CAbs) of 250 nm
BC from λ = 550 to 840 nm. The dashed black line represents AAE fit
of the experimental data over full range. The solid red line represents
the calculated absorption spectrum utilizing the T-matrix method
described in the text. Measured results are reported as experimental
mean and 2σ uncertainty propagated across all measurements.
CAbs is linear for pure BC particles, we expect the BC portion of
the internally mixed particles to behave similarly.
A better resolved absorption spectrum (Δλ = 50 nm) for BC
particles with a 250 nm mobility diameter corresponding to a
BC mass of 6.3 fg is shown in Figure 2c. Here the data has been
plotted as CAbs versus wavelength, instead of CAbs versus mass as
in Figure 2b, as absorption depends only upon mass for all
measured wavelengths (i.e., linearity of traces in Figure 2b).65
The MAC (defined as CAbs/mp) at 550 nm is 7.89 ± 0.25 m2
Figure 3. TEM images of 250 nm (a) AS/BC, (b) NaCl/BC, and (c)
HA/BC. The scale bar corresponds to 100 nm.
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is based on eq 1 with CAbs of the BC core calculated from the
particle mass, BC mass fraction and MAC of 250 nm particles:
determined by utilizing the electron beam from the TEM to
melt and desorb the embedding material and reveal the particle
interior, showing the presence of BC aggregates (see
Supporting Information Figure S6 for images showing time
series of embedding material melting). For NaCl/BC,
aggregates were observed both internal and external to the
embedding material.
Absorption of BC Embedded in Nonabsorbing
Materials. Below we focus on how mixing state influences
particle absorption. We first concentrate on the optical
properties of BC mixed with nonabsorbing materials. Figure
4a shows the measured CAbs spectra spanning between λ = 550
EAbs, λ =
CAbs,embedBC, λ
mpχBC MACBC, λ
(6)
At the shortest wavelengths, EAbs is within 2σ for both the AS/
BC and NaCl/BC systems. Toward the long-wave side of the
spectrum, EAbs values for both systems differ as NaCl/BC has
remained nearly constant and AS/BC has decreased.
Wavelength dependent EAbs have been calculated previously
for embedded BC using spherical particle Mie theory,17 but to
our knowledge the AS/BC data represent the first measured
spectra showing a wavelength dependent EAbs for embedded
BC; the NaCl/BC EAbs is wavelength independent across the
measured spectral range. We used the T-matrix method to
investigate if the method is able to capture the measured
spectral dependence for each embedding material. Calculations
were run for fully embedded and half-embedded BC. The
simulated enhancement spectra are shown in Figure 4a for each
case where the lower (dashed) and upper (solid) bounds
represent the half-embedded and the fully embedded cases,
respectively. The region in between these two values represents
mixing states migrating from half-embedded to fully embedded
BC and assumes that the BC aggregate’s center of mass
overlaps that of the host sphere. Moving BC aggregates off
center results in less than 3% decrease in absorption cross
section, consistent with previous calculations.26 The corresponding simulated and measured AAEs are shown in Table 1.
Table 1. Measured and Modeled AAEs for Half and Fully
Embedded AS/BC and NaCl/BCa
calculated AAE
AS/BC
NaCl/BC
a
measured AAE
half embedded
fully embedded
1.43 ± 0.05
1.34 ± 0.06
1.36
1.32
1.45
1.41
Propagated uncertainties are shown along with the fit AAEs.
Absorption of BC with Absorbing Material. A weakly
absorbing BrC surrogate material was isolated from humic acid
extracts as described in the Materials and Methods. Upon
atomization and drying, pure HA forms spherical particles with
an average effective density of 1.5 ± 0.1 g cm−3, as determined
from the mass-mobility scaling relationship (Dfm = 3.10 ±
0.04). The RI of HA was determined to be (1.58 ± 0.01) +
(0.02 ± 0.01)i at λ = 660 nm. For T-matrix calculations, we
assumed the real component of RI was invariant with
wavelength and the imaginary part of the RI is well described
by the absorption spectrum of the material in aqueous solution
(see the Supporting Information for HA refractive index
determination and aqueous absorption spectrum). The
absorption spectrum of HA aerosol is shown in Figure 5a,
and exhibits an AAE of 5.31 ± 0.14. The MAC of HA aerosol
was 0.85 ± 0.06 m2 g−1 at 550 nm, nearly an order of
magnitude lower than that of BC at the same wavelength, and is
comparable to strongly absorbing BrC in prior work.71
At an χBC = 0.13, CAbs increases by a factor of 2 to 3 relative
to the pure HA at the same mobility diameter. The AAE of BC
embedded in HA is 1.91 ± 0.05, lower than that of pure HA
but higher than that of BC embedded in AS or NaCl. For
modeling we adopted a single sphere model for the pure HA
and a fully embedded model for HA/BC particles, consistent
Figure 4. (a) Measured absorption cross section (CAbs) spectra of AS/
BC (red circles) and NaCl/BC (blue squares) for 250 nm mass
selected particles (χBC = 0.13) from λ = 550 to 840 nm. Lines
represent fully embedded (solid) and half embedded (dashed) BC for
each system; the shaded regions represent mixing states in between
these two extremes. (b) EAbs for AS/BC (red circles) and NaCl/BC
(blue squares). Measured results are reported as experimental mean
and 2σ uncertainty propagated across all measurements.
and 840 nm for particles that were size and mass selected at Dp
= 250 nm and mp = 13.50 ± 0.06 and 11.48 ± 0.06 fg for AS
and NaCl, respectively with χBC = 0.13. The absorption cross
section is higher for the fully embedded in AS versus the
partially embedded NaCl. The spectra of the embedded
particles have higher AAEs than bare BC; 1.43 ± 0.05 for
AS/BC and 1.34 ± 0.06 for NaCl/BC versus 1.03 ± 0.09 for
bare BC. The absorption enhancement of embedded BC (EAbs)
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factor of 3 at the shortest measured wavelengths and is 1.6 in
the near-IR. Removing HA absorption from EAbs,λ shows that
much of the enhancement at short wavelengths is a result of
HA absorption as seen by the relatively constant EAbs‑HA,λ in
Figure 5b. This trend was first predicted in prior calculations
using spherical particle Mie theory, where the data are similar in
shape and magnitude to 300 nm absorbing core with a weakly
absorbing 100 nm thick embedding.17
■
DISCUSSION
In this investigation the mass mixing ratio and mobility
diameter of internally mixed BC and NaCl, AS or HA particles
were held constant at χBC = 0.13 and 250 nm, respectively. The
primary variables that differ between the embedding materials
are density, refractive index and how each material interacts
with BC under the conditions of aerosol generation to form
particles (mixing state). Below we highlight how each variable
impacts EAbs,λ using the simplest case of spherical particles. Due
to density differences between the embedding materials, the χBC
corresponds to a 133, 127, and 120 nm BC core embedded in
NaCl, AS and HA, respectively. Using a core−shell model in
conjunction with Mie theory to calculate EAbs,λ for each of the
nonabsorbing core/shell diameters results in an EAbs,λ that is 4%
± 1% higher for NaCl/BC than AS/BC over the 550 and 850
nm wavelength range studied here (see Supporting Information
Figure S7). The refractive index of the coating material also
affects the calculated EAbs,λ. This is best shown by keeping the
BC core diameter constant (127 nm) and varying the coating
refractive index, where Eabs,λ increases an additional 0.5% to 1%
for NaCl/BC over AS/BC. Thus, for equivalent 250 nm
particles using an idealized spherical core−shell model, the
EAbs,λ for NaCl/BC is calculated to be 5% to 6% higher than
AS/BC. TEM images show that for NaCl/BC BC is partially
embedded in NaCl, whereas BC is fully embedded by AS.
Despite these differences, the measured EAbs,λ are equivalent at
the shortest measured wavelength region and is up to 15%
higher in the near-IR for NaCl/BC, suggesting that using
simple core−shell spherical models do not adequately capture
EAbs,λ for embedded particles.
The spectral shape and wavelength dependence of CAbs can
also be described by the measured AAE when compared to
pure BC. The AAE for NaCl/BC and AS/BC are within the 2σ
measurement uncertainty, see Table 1, indicating that for BC
mixed with nonabsorbing materials the AAE is near 1.4 and
independent of particle mixing state. For HA/BC the AAE is
nearly 2. Prior modeling studies of BC embedded in absorbing
and nonabsorbing materials have classified the AAE into two
regimes to help better describe measured ambient aerosol,
where BC embedded with nonabsorbing materials have an AAE
< 1.6, whereas only BC embedded with an absorbing materials
have AAE > 1.6.17 Although this study covers a small parameter
space of the possible aerosol combination of BC core
diameters, core−shell ratios, and BrC imaginary RI values,
the presented data support this classification.
Figure 5. (a) Measured absorption cross section (CAbs) spectra of pure
HA (gold triangles) and HA/BC (blue diamonds) with χBC = 0.13 for
250 nm size-mass selected particles from λ = 550 to 840 nm. Lines
represent calculated CAbs for each system. (b) EAbs for HA/BC
including HA absorption (solid blue diamonds) and only including
enhancement from HA after subtraction HA absorption (open blue
diamonds); see eq 7 in discussion and text. Lines represent calculated
EAbs using fully internal mixing model for both definitions. Measured
results are reported as experimental mean and 2σ uncertainty
propagated across all measurements.
with TEM images of BC embedded in HA. The T-matrix
calculated CAbs spectrum agrees well with the experimental
results in each case, as shown by the solid lines in Figure 5a.
The EAbs,λ of BC embedded in HA was calculated using two
methods. Using eq 1, EAbs,λ includes the impact of HA
absorption and enhancement of BC absorption. For the second
method, the inherent relative contributions to absorption from
HA is removed by redefining EAbs,λ as
EAbs‐HA, λ =
■
CAbs,embedBC, λ
(1 − χBC )mp MACHA, λ + χBC mp MACBC, λ
ASSOCIATED CONTENT
* Supporting Information
S
(7)
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.est.6b01473.
Additional information regarding drop cast TEM images
from a BC solution, determination of the refractive index
of BC and BrC at λ = 660 nm using a cavity ring-down
where MACHA,λ represents the MAC of the pure HA. Note that
EAbs‑HA,λ when applied to BC embedded in HA, describes the
EAbs arising only from BC absorption enhancement and reduces
to eq 6 when applied to the nonabsorbing material. Both results
are shown in Figure 5b. The EAbs,λ shows an increase up to a
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Article
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spectrometer, the mass-mobility scaling relationship of
BC and HA, the modeling parameters used T-matrix
calculations, a comparison of Mie theory and T-matrix
calculations for homogeneous and coated spheres, a
comparison of BC MAC from Figure 2b and c, and TEM
images of particle melting and the influence of core−shell
size and refractive index on absorption enhancement
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Phone: (301)975-8709. Fax: (301)
975-3670.
Notes
The authors declare no competing financial interest.
■
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